Si

d to%, 1ertestscesSireyon aloys

2011 Elsevier Ltd All rights reserved.

1. Introduction

Among the materials of the tribologiceutectic AlSi alloys preferentially haveattention to wear related applications such

kers ancient,ical prwith

to Al m

(b) better mechanical properties compared with AlSi12CuMgNi alloy at the medium high temperature and (c) low cost by usingAl, Mg, Si as starting materials [915]. However, the mechanical

2. Experimental details

Commercially pure 12 wt.% Si containing Al ingot alloy and pureMg were used as starting materials to prepare the Al12SiXMg al-loys. The Mg contents varied between 0 wt.% and 20 wt.% in the Almatrix. The Mg was added to the Al12Si alloy melt at 800 C alongwith 0.05% of Sr, 0.20% of red phosphorous and 0.30% ofNaCl + 30MgCl2 + 10KCl mixtures necessary for the renementand modication of precipitations which were all preheated at

Corresponding author. Address: Karabuk niversity, Engineering Faculty,Metallurgy and Materials Engineering, Karabuk, Turkey. Tel.: +90 370 4332021;fax: +90 370 4333290.

E-mail addresses: ysun@karabuk.edu.tr (Y. Sun), hahlatci@karabuk.edu.tr,

Materials and Design 32 (2011) 29832987

Contents lists availab

Materials an

elshahlatci@hotmail.com (H. Ahlatci).In situ composites are multiphase materials where the reinforc-ing phase is synthesized within the matrix during composite fabri-cation. This conicts with ex situ composites where the reinforcingphase is synthesized separately and then inserted into the matrixduring a secondary process such as inltration or powder process-ing. In situ processes can create a variety of reinforcement mor-phologies, ranging from discontinuous to continuous, and thereinforcement may be either ductile or ceramic phases [8].

AlMg2Si in situ composite offers attractive advantages, as acandidate material, in future industrial applications. These advan-tages include (a) weight reduction due to the low density of Mg2Si;

standing, most of our previous studies were focused on theimprovement of microstructure and mechanical properties of theas-cast composites [1618].

A great number of researches are available in the literature[13,1822], in relation to the preparation and processing of thein situ composites containing Mg2Si particles as the reinforcementmaterials. However, reports on the wear behavior of the in situcomposites appear rarely. Therefore, this study deals with themechanical and wear behaviors of the hot extruded in situ Mg2Sireinforced Al12Si alloy matrix composites.engines, pistons, liners, pulleys, rocdensity and thermal expansion coefness, ambient temperature mechanstrength) and wear resistance alongcan be achieved with addition of Si0261-3069/$ - see front matter 2011 Elsevier Ltd Adoi:10.1016/j.matdes.2011.01.009al importance, hyper-received considerableas internal combustiond pivots. Reduction inimprovement in hard-operties (modulus andan excellent castabilityatrix [17].

properties of the as-cast alloys containing Mg2Si phases were notsatisfactory due to the brittle matrix and large Mg2Si phase[11,14]. For further improvement of the mechanical properties, ra-pid solidication and mechanical alloying have been used to pro-duce the material with very ne matrix structure and in situMg2Si particles. Unfortunately, such techniques are too expensiveand too complex to be accepted by the engineering communityfor general applications. Therefore, preparing the in situ compositeby simple casting process seems to be the most hopeful routewhen facing further commercial demands. Based on this under-alloy.Short Communication

Mechanical and wear behaviors of Al12with in situ Mg2Si particles

Yavuz Sun, Hayrettin Ahlatci Karabk University, Engineering Faculty, Karabuk 78050, Turkey

a r t i c l e i n f o

Article history:Received 21 September 2010Accepted 5 January 2011Available online 15 January 2011

a b s t r a c t

A study has been conducte(where Mg contents were 5mixtures. Mechanical proptests. Reciprocating wear ton the unlubricated surfaXMg alloys consisted of theshape dark particles and gstrated that volume fracticontent. Mg containing al

journal homepage: www.ll rights reserved.XMg composites reinforced

investigate the mechanical and wear behaviors of the Al12SiXMg alloys0% and 20%) cast by adding modiers such as Sr, red phosphorous and salties were determined by hardness measurements, compression and wearwere conducted by rubbing ceramic (Al2O3) and steel (AISI 52100) balls

by applying the normal load of 2 N. The microstructure of the Al12Sineedles and Mg2Si precipitates (two morphologies of which are polyhedralcolored components of Chinese script) in the Al matrix. Results demon-nd size of the primary Mg2Si particles increased with increasing the Mgexhibited higher hardness and better wear resistance than the Mg freele at ScienceDirect

d Design

evier .com/locate /matdes

300 C before charging into the melt. In order to balance the oxida-tion loss, the above additions were charged with an extra of15 wt.%. Degassing was conducted by using C6Cl6 before the cast-ing. After cleaning the slag, the melt was poured at 800 C into achill mould to produce the rod shaped ingots of 20 mm in diameterand 300 mm in length. After the casting, the rods were extruded di-rectly to 16 mm diameter (corresponding to extrusion ratio of 1.6)at 300 C. Chemical compositions of the investigated alloys deter-mined by optic emission methods are given in Table 1.

Characterizations of the extruded rods were carried out interms of mechanical and wear tests as well as microstructuralexaminations. Microstructural survey was conducted via an opticallight microscope (LOM) after preparing the samples according to

20 mm of length and 10 mm of diameter were tested by a DartecUniversal testing machine at a crosshead speed of 0.5 mm/min todetermine the compression behavior of the alloys. The results ofthe compression tests were compiled by taking the average ofthe decisiveness of ve specimens.

Wear performances of the examined alloys were evaluated innormal atmospheric condition (20 1 C and 45 5% RH) by utiliz-ing a reciprocating wear tester which was designed according toASTM G133 standard. The reciprocating wear tests were carriedout throughout the total testing time of 100 min by applying thenormal load of 2 N to the unlubricated surfaces of the samples with10 mm diameter ceramic (Al2O3-1577 HV0.5) and steel (AISI 52100-55 HRC) balls in which average surface roughness (Ra) values were0.2 lm and 0.1 lm, respectively. In this study, the sliding speed ofthe balls on the surfaces of the samples was 0.02 m/s for a totalsliding distance of 120 m. After the wear test, the samples werecleaned with alcohol and proles of the wear tracks were recordedby Dektak 6M stylus prolometer. Results of the wear tests werequantied in the unit of mm2, by considering the area of the weartrack calculated from the 2-D prole. After the wear test, weartrack of the investigated alloys was determined by a scanning elec-tron microscope (SEM).

3. Results and discussion

Table 1The chemical composition of the investigated alloys.

Alloys Si(%)

Mg(%)

Fe(%)

Cu(%)

Mn(%)

Zn(%)

Ti(%)

Al(%)

Al12Si0Mg

13.5 0.1 0.6 0.1 0.4 0.1 0.15 85.1

Al12Si5Mg

11.3 5.2 1.5 0.2 0.3 0.1 0.05 81.7

Al12Si10Mg

11.1 11.0 1.7 0.2 0.2 0.1 0.05 76.7

Al12Si20Mg

12.2 19.1 2.1 0.3 0.3 0.1 0.05 65.9

(b)

2984 Y. Sun, H. Ahlatci /Materials and Design 32 (2011) 29832987standard metallographic procedures. Volume fractions and averagesizes of the phases present in the microstructures of the investi-gated alloys were quantied by using the line intercept method.The phases were identied with Cu Ka on an X-ray diffractometer.

Room temperature mechanical properties of the alloys weredetermined by hardness measurements and compression tests.Hardness survey of the investigated alloys was measured on Shi-madzu HMV2 microhardness tester by applying indentation loadof 2000 g with a Vickers indenter. At least, ten successive measure-ments were performed for each condition. Round specimens with

50 m

(a) 50 m

(c)

Fig. 1. LOM micrographs of the (a) Mg free, (b) 5 wt.% Mg,50 m

50 m

(d)Microstructures of the extruded Al12SiXMg alloys are shownin Fig. 1. The microstructures were mainly composed of needleshaped light grey colored phases, polygonal shaped dark particles,Chinese script and white colored matrix. Porosities were not ob-served in the microstructures. Considering the results of the XRDanalysis presented in Fig. 2, the microstructural constituents wereidentied as Si (needle shaped light grey color phases), Mg2Si andAl (white colored matrix). Mg2Si appeared in two different mor-phologies (i.e. polyhedral shaped dark particles and grey coloredChinese script phases) as mentioned in the literature [13,1822].(c) 10 wt.% Mg and (d) 20 wt.% Mg containing alloys.

itated in the matrix linearly increased while the volume fraction ofthe Si needles decreased. The microscopic examinations also re-vealed that, in the investigated alloys extruded at the ratio of 1.6.Mg2Si components of the Chinese script and Si needles were re-ned (Fig. 1b and c) and also the morphology of the primary Mg2Siparticles became irregular by increasing the Mg content [26]. Asseen in Fig. 1d, to generate the primary and Chinese script Mg2Si,almost all of Si element reacted with Mg [27] in the Al12Si20Mg alloy;

2Mg Si Mg2Si 1Therefore free Si needles did not observe in the microstructure ofthe Al12Si20Mg alloy.

The results of the room temperature mechanical tests againstthe Mg content of the investigated alloys are charted in Fig. 4. Anincrease in the hardness and compression strength was observedto be derived from the increase in the Mg content and decreasein the Si needles of the alloys, which is attributed to the incrementof the primary Mg2Si volume fraction and size. The addition of the10 wt.% Mg yielded the hardness value of 80 HV2, which is 30%higher than the hardness of the Mg free alloy. Further addition of

20 30 40 50 60 70 80 90 100Angle (2 )

Inte

nsity

Si2

Al

Mg Si

(a)

(b)

(c)

(d)

Fig. 2. XRD patterns of the (a) Mg free, (b) 5 wt.% Mg, (c) 10 wt.% Mg and (d) 20 wt.%Mg containing alloys.

0

40

80

120

160

200

Bul

k H

ardn

ess (

HV ) 2

(a)

Y. Sun, H. Ahlatci /Materials and Design 32 (2011) 29832987 2985Since the compositions of the investigated alloys were hyper-eutectic, it is suggested that the polyhedral dark particles, whichwere identied as primary Mg2Si particles, were in situ formedduring initial stage of the solidication process. Although the for-mation mechanism of the primary Mg2Si particles has not beenclearly explained, the addition of the modiers such as Sr, redphosphorus and salt mixtures to the melt may result in an increasein the number of nuclei and change the morphology and size of theprimary Mg2Si particles [18,2225] by suppressing their aniso-tropic growth by modifying both the solidliquid interfacial energyand surface energy of the solid Mg2Si phase or poisoning the sur-face of the in situ Mg2Si nuclei owing to the segregation of Na orK at the liquidsolid interface [18,2022].

Details of microstructural examinations revealed that the pri-mary Mg2Si particles were homogeneously distributed in micro-structures (Fig. 1). The variation of the size of the primary Mg2Siparticles, the volume fraction of the primary Mg2Si and Si particlesagainst the Mg contents is seen in Fig. 3. Upon increase of the Mg,

the volume fraction and size of the primary Mg2Si particles precip-

0 5 10 15 20Mg Content (wt. %)

0

10

20

30

40

50

Part

icle

Volu

me

Frac

tion

(%)

0

10

20

30

40

50

Part

icle

Siz

e (

m)

SymbolVolume Fraction of The Primary Mg2Si Size of the Primary Mg2Si Volume Fraction of the Si

Fig. 3. The effect of Mg content of the alloy on the volume fraction and size of theprimary Mg2Si and Si particles.

0 5 10 15 20

Mg Content (wt. %)

0 5 10 15 20

Mg Content (wt. %)

300

400

500

600

Com

pres

sion

Stre

ngth

(MPa

)

(b)Fig. 4. The effect of Mg content on the (a) hardness and (b) compression strength ofthe investigated alloys.

the Mg resulted in severe hardening. The hardness of the Al12Si20Mg alloy was almost two times of that of the Al12Si10Mg al-

alloys. The ceramic ball caused relatively higher wear loss owing toits higher hardness as compared to the steel ball. The decrease ofthe wear loss with increasing the Mg content of the alloys is attrib-uted to the strengthening of the alloys by the precipitation of theprimary Mg2Si particles. In the case of the Al12Si20Mg alloy,better protection of the matrix (Fig. 4), worn by both the ceramicand steel balls, was observed probably due to more coarser Mg2Siparticles [3034] as compared to the other alloys studied. The wearbehavior of the composites is affected by various parameters suchas type, size and volume fraction of the reinforcement. In mostcases the wear resistance increased with increasing size of rein-forcements [27,28,33,35,36]. However it has not been improved(in some cases it was even weakened) when the wear took placeby delamination and or particle fracture mechanism. Generallythe wear resistance of composites has been improved by increasingthe volume fraction of the reinforcements [27].

SEM micrographs of the wear tracks developed on the surfacesof the investigated alloys during dry sliding wear tests using Al2O3and steel balls are given in Fig. 6. Dry sliding wear generated thewear tracks having the characteristics of abrasive and mostly adhe-sive wear surface appearance (Fig. 6). Therefore, the steel ball pro-vided the adhesive wear characteristics on the alloys, while theceramic ball caused abrasive wear. SEM micrographs of the alloyswhich were worn by steel ball show regions from which the mate-rial brake off while Al2O3 ball made clear grooves on the surface ofthe alloy (Fig. 6). It was seen that the magnitude of the damage onthe alloy surface decreased as the Mg content of the alloy

alloy, which has varying Mg contents between 0 and 20 wt.% in the

0 5 10 15 20Mg Content (wt. %)

0.00

0.01

0.02

0.03

0.04

0.05W

ear

Trac

k A

rea

(mm

)

CounterfaceType Symbol Ceramic ball Steel ball

2

Wear Track Area

Fig. 5. The effect of the Mg content on the area of the wear tracks developed byceramic and steel balls.

2986 Y. Sun, H. Ahlatci /Materials and Design 32 (2011) 29832987loy. The increase in compression strength is continual along withincrease in the Mg content. The mechanism of hardening wasbelieved to be due to the combination of solid solution and forma-tion of the in situ particles being obstacles to the movement of dis-locations [28,29].

The results of the wear tests conducted by rubbing the ceramicand steel balls on the surface are described in Fig. 5. The addition ofthe Mg to the investigated alloys reduced the area of the wear track(henceforth referred as wear loss) developed on the surface of the(a)

500 m

(c)

500 m

Fig. 6. SEM micrographs of the inner regions of the wear tracks produced on (a) Mg free(d) 20% Mg worn by Al2O3 ball.Al matrix.

(b)

500 m

(d)

500 m increased.

4. Conclusion

The following conclusions can be drawn from the results of thepresent investigation conducted on the hot extruded Al12SiXMgworn by steel ball, (b) 20% Mg worn by steel ball and (c) Mg free worn by Al2O3 ball,

1. The microstructure of the Al12SiXMg alloys consisted of theSi needles and Mg2Si precipitates (as primary polygonal precip-itates and as a component of Chinese script) in the Al matrix.The volume fraction and size of the primary polygonal Mg2Siparticles increased as long as the Mg content of the alloyincreased.

2. The Mg addition increased the bulk hardness, compressivestrength and wear resistance of the Al12SiXMg alloys, whichis attributed to the increase of the Mg2Si precipitation.

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Mechanical and wear behaviors of Al12SiXMg composites reinforced with in situ Mg2Si particlesIntroductionExperimental detailsResults and discussionConclusionReferences